transaction: Add a missing error return
transaction: Add a missing error return

When trying to uninstall a non-installed app, we run
into an assertion, because the code forgets to return
after setting the expected error.

Found while writing tests.

Closes: #2244
Approved by: alexlarsson

matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak